The present invention relates to a refrigerating system in which a fluid other than fluorocarbon can be used as a refrigerant.
In recent decades, it is reported that emissions of various chemicals including fluorocarbon (particularly, chlorofluorocarbon often abbreviated to CFC) will deplete a protective ozone layer surrounding the earth. As one of approaches to reduce such hazardous emissions, development is made of a refrigerating system using a substitute refrigerant instead of a fluorocarbon refrigerant.
For example, U.S. Pat. No. 5,245,836 discloses a refrigerating system using carbon dioxide as a refrigerant. In this refrigerating system, carbon dioxide is supplied to a compressor to be compressed into a supercritical state having a pressure higher than its critical pressure. Then, carbon dioxide in the supercritical state is cooled by a radiator, decompressed or reduced in pressure by an expander, and evaporated by an evaporator. During evaporation, heat is absorbed from an external fluid such as air as latent heat of evaporation. Thus, desired refrigeration is carried out.
The "suporcritical state" mentioned above is a state in which each of the temperature and the pressure of carbon dioxide is equal to or higher than its critical point, in other words, a state in which carbon dioxide molecules are moving around like in a gaseous phase although the density of carbon dioxide is substantially equal to a liquid-phase density. As will be understood from the Mollier diagram, the critical temperature of carbon dioxide is about 31.degree. C. and the critical pressure corresponding thereto is about 7.38 MPa (mega pascal).
As described above, the critical temperature of carbon dioxide is about 31.degree. C. which is considerably low as compared with a halogenated refrigerant (for example, 112.degree. C. for R12 (=CCl.sub.2 F.sub.2)). Therefore, in a high-temperature environment such as in a summer season, the temperature of the outside air may become higher than the temperature of carbon dioxide as the refrigerant so that the temperature of carbon dioxide at an outlet of the radiator is higher than 31.degree. C. In such a situation, the radiation of carbon dioxide at the radiator is decreased and, as a result, the heat exchange at the evaporator is decreased. Thus, it is difficult to achieve a desired refrigerating effect. In view of the above, in a refrigerating circuit using carbon dioxide as a refrigerant, an outlet pressure of the radiator is set at a high level to remove the above-mentioned disadvantage. Such a high outlet pressure of the radiator can be obtained by increasing a discharge pressure of the compressor.
In the meanwhile, the amount G of the refrigerant circulating through the refrigerating circuit (hereinafter referred to as the circulating amount) is given by: EQU G=(q.multidot..eta.)/v,
where q represents a displacement by a piston of the compressor, .eta., a volumetric efficiency, and v, a specific volume of the confined refrigerant. Therefore, as far as the compressor is not exchanged, the circulating amount G is inversely proportional to the specific volume of the confined refrigerant. It is noted here that the specific volume of carbon dioxide is smaller than that of the halogenated refrigerant. Accordingly, the circulating amount G of the refrigerating circuit using carbon dioxide as the confined refrigerant is great as compared with the case where the halogenated refrigerant is used. For example, the circulating amount G for carbon dioxide is about 5.5 times and about 7 times as great as those for R12 and R134a (=CH.sub.2 FCF.sub.3), respectively. This means that, in the refrigerating circuit using carbon dioxide as the confined refrigerant, the refrigerating ability equivalent to that achieved by the halogenated refrigerant can be assured even when the circulating amount G is about 1/5.5 and about 1/7 as small as those for R12 and R134, respectively. The circulating amount G of such a small value decreases a pressure loss in a refrigerant pipeline. Therefore, the refrigerant pipeline is allowed to have a small inner diameter as compared with that of the refrigerating circuit using the halogenated refrigerant. For example, the inner diameter of the refrigerant pipeline can be reduced to about 1/4 as compared with the case where R134a is used.
As described above, the inner diameter of the refrigerant pipeline will be reduced by the use of carbon dioxide as the refrigerant. However, it is still necessary to provide an arrangement for avoiding the wear and the damage of various parts of the compressor, i.e., sliding surfaces of a piston and a piston cylinder and sliding portions of bearings. For this purpose, a lubricating oil is confined in the refrigerating circuit in addition to carbon dioxide as the refrigerant. The lubricating oil circulates through the refrigerating circuit to lubricate the sliding surfaces and the sliding portions.
In case where the refrigerant pipeline has a small inner diameter, sticking of the lubricating oil onto an inner surface of the refrigerant pipeline will result in a considerable increase in pressure loss. Under the circumstances, the inner diameter of the refrigerant pipeline is inevitably increased even if carbon dioxide is used as the refrigerant. Thus, it is difficult to achieve an advantage which would be obtained by the use of carbon dioxide as the refrigerant, i.e., to achieve reduction in size of the refrigerating system.